The proposed research is concerned with the fidelity of translational elongation and termination in the model eucaryote Saccharomyces cerevisiae. The role of soluble translation factors and other proteins in the fidelity of genetic code translation will serve as a central theme in this work. Mutants affecting elongation and termination will be used to identify and establish the roles of genes and proteins important in the fidelity process. The major focus of the proposal will be on those aspects of mistanslation unique to eucaryotes in which fidelity is viewed as a central parameter determining cellular growth rate. In order to provide continuity, some features of the translational fidelity apparatus shared in common by procaryotes and eucaryotes will also be assessed. First, we propose to study three translational suppressor genes, suf12, suf13, and suf14, that are known or suspected to encode novel fidelity proteins that appear to have no functional procaryotic counterpart. The study of these genes and their products will aid in assessing the view that complex, compartmentalized eucaryotic cells may require a more sophistocated level of control over fidelity than that required in procaryotes. The second part of the proposal addresses the process of translational termination for which very little information is available in eucaryotes. Termination factors and ribosome components important in the termination process will be characterized. Information from these studies will be used to assess and apparent relationship between mistranslation and termination. Finally, the tRNA selection and proof-reading system will be studied in yeast in relation to two types of misreading, amino acid misincorporation and frameshifting. This work will focus on the function of the known soluble factors EF-1 alpha (responsible for tRNA binding to the ribosome) and EF-2 (responsible for translocation). Direct information on tRNA selection and proof-reading in yeast will obviate the need for indirect inference to procaryotic systems and will help assess aspects of mistranslation unique to eucaryotes.